CUDA compilation failed
With *buntu 20.04, modify the blender.desktop launcher to:

CYCLES_CUDA_EXTRA_CFLAGS="-ccbin clang-8" blender


As per the Blender web page as of 07-April-2020, Blender is not compatible with gcc 4.7 or higher.

The above command will launch Blender with compiler settings compatible with 20.04. The 1st GPU render requires a few minutes to compile the CUDA renderer, but afterwards renders will run immediately. Nvidia 1080 CUDA rendering was 6× faster than my old 4771 CPU alone.